FIG. 21 is a diagram showing a construction of a prior art optical modulator provided with a distributed feedback laser (hereinafter referred to as DFB laser) having a light absorbing layer of multi-quantum well structure. In the figure, reference numeral 1 designates a DFB laser having a resonator length of 300-600 .mu.m, a width of 300 .mu.m, and a thickness of 100 .mu.m. Reference numeral 2 designates an optical modulator provided with the DFB laser 1, having a length of 100-300 .mu.m, a width of 300 .mu.m, and a thickness of 100 .mu.m.
In the DFB laser 1, a positive side electrode 3 is provided as an electrode of the laser diode 1. An electrode 4 is provided as a common grounding electrode for the DFB laser 1 and the optical modulator 2. An active layer 6 is provided as active layer of the DFB laser 1.
In the optical modulator 2, a light absorbing layer 7 comprises a multi-quantum well structure including twelve quantum well layers 4-5 nm thick, putting between barrier layers 8-10 nm thick, and its total thickness is 140 nm. An optical modulator negative side electrode 8 is an electrode for inputting a modulation signal to the optical modulator 2. A signal voltage source 9 supplies a modulation signal to the optical modulator negative side electrode 8. Reference numeral 10 designates a modulated signal light that is obtained by modulating the output light from the DFB laser 1 by the modulation signal that is output from the signal voltage source 9.
FIGS. 22(a) to 22(d) are diagrams for explaining the operation of the optical modulator 2, where FIG. 22(a) shows the change in the modulation signal voltage which is applied to the optical modulator 2. FIG. 22(b) shows the change in the modulated signal light output, occurring when the modulation signal is applied, and FIG. 22(c) shows the change in the refractive index of the light absorbing layer 7 of the optical modulator 2, also occurring then. FIG. 22(d) shows the wavelength chirping (frequency variation) occurring in the optical modulator 2.
A description is given of the operation of the optical modulator.
First of all, when a D.C. current is flown in the DFB laser 1 as a light source from the LD positive side electrode 3 to the common grounding electrode 4, a continuous wave (CW) laser light is generated in the active layer 6 and the CW laser light transmits in the active layer 6 toward the optical modulator 2. Then, the CW laser light advancing toward the light absorbing layer 7 of the optical modulator 2 is incident to the optical modulator 2.
The light absorbing layer 7 is of a multi-quantum well structure, and when no voltage is applied between the common grounding electrode 4 and the optical modulator negative side electrode 8 of the optical modulator 2, the light absorbing layer 7 makes the CW laser light incident to the light absorbing layer 7 transmit therethrough, while when a voltage is applied between the common grounding electrode 4 and the optical modulator negative side electrode 8, the light absorbing layer 7 absorbs the CW laser light only during when a voltage bias is applied, due to the quantum confining Stark effect.
In other words, when a voltage of modulation signal in a waveform as shown in FIG. 22(a) is applied to the optical modulator negative side electrode 8, during a period when the modulation signal voltage is at a negative level, the light absorbing layer 7 absorbs the CW laser light, whereby no laser light appears at the output side thereof, and the modulated signal light output is turned off. On the other hand, during when the modulation signal voltage is at 0 voltage level, the CW laser light transmits the light absorption layer 7, whereby the modulated signal light output becomes on. Then, the waveform of the modulated signal light 10 becomes a waveform as shown in FIG. 22(b) in accordance with the variation in the modulation signal voltage.
In the prior art optical modulator constituted as described above, the quantum confining Stark effect of the multi-quantum well structure light absorbing layer in an absorbing type optical modulator is of a nature that the absorbing quantity generally increases when an electric field is applied. As the light absorbing quantity increases, the refractive index in the light absorbing layer increases as shown in FIG. 22(c) due to the relation of Kramers-Kroning, and the refractive index again decreases when no electric field is applied. With the refractive index varies, i.e., with the quantity of transmission/absorption varies, the optical length varies and the wavelength of the modulated signal light, that is output from the optical modulator output facet varies, thereby resulting chirping as shown in FIG. 22(d). The quantity .DELTA..lambda. of this wavelength chirping is about several .ANG. for the wavelength of 1.55-1.56 .mu.m, the modulator of which wavelength is generally used because it produces the least transmission loss. When this wavelength chirping is large, the optical signal is subjected to distortion through the optical fiber transmission, thereby resulting in difficulty in performing a high speed long distance transmission.